There are two methods of refreshing the finish on a baking pan, retinning and reglazing, and this document will discuss whether these processes qualify as some form of kashering.

Retinning

When the tin wears away on a tin pan or a tin-coated pan, an additional layer of tin is added either to the worn area or to the whole pan. The process involves rubbing the area to be coated with acid (to help the new tin adhere better), heating tin to its melting point of 450° F, pouring (or otherwise applying) the molten tin to the pot-surface, and then putting the pot into cold water to set the fresh tin in place.1

Magen Avraham 451:27 says that the tin which is hot enough to be liquefied then that is well beyond what is required for libun gamur.2 What about the underlying pan? Has it been kashered with libun gamur? Clearly, the pan onto which the tin is being applied is not as hot as the liquid tin, or else it would melt; so just because the liquefied tin is undergoing libun gamur does that mean that the pan being retinned is also undergoing that same level of kashering? In fact, Magen Avraham describes retinning and says that a pan which has undergone this process has been kashered via libun kal,3 and Pri Megadim ad loc. says that it is פשיטא that this process does not qualify as libun gamur.4

Lastly, it is worth mentioning that Mishnah Berurah 451:77 makes two points regarding this process:

One should not rely on retinning as any form of kashering unless he can be certain that all surfaces of the utensil were retinned.

If a non-Jew retinned a Jew’s pan, the pan should undergo tevillas keilim again.5

Reglazing

Baking pans are made of metal which is coated with silicon to prevent the product from sticking to the pan. Pans which are used commercially require a fresh coating of silicon (i.e. reglazing) every few months. The process of reglazing involves a considerable amount of heat and chemicals which are used to remove the old glaze and any residue, and put on a fresh coat.

In order to better understand this process and evaluate its suitability for kashering, Rabbi Oppenheimer and I visited a large reglazing facility which is part of nationwide network of similar companies, all of whom use the same process. While we were asked not to reveal the exact details of this company’s process (which they tell us is basically used in the entire industry), we can report that reglazing is a 7-9 hour process which can be roughly divided into three parts – stripping, glazing, and drying, as follows:

Stripping uses different chemicals, acids and soaps to strip away the existing silicon layer together with any residue of food, grease, and burnt product. Most of the stripping is done at ambient temperature and there are no stages where hot pure water is used. However, during one stage the liquid agents being used are very close to 212° F and at another they are at about 140-160° F, such that these stages may qualify as hag’alah with sha’ar mashkim (which b’dieved is acceptable).6

The glaze is cured/dried by placing a stack of pans into a hot chamber which is (on the low end of temperatures that are) hot enough to qualify as libun kal if we can be sure that (a) the pans stay in for long enough and (b) even the pans in the center of the stack are heated sufficiently.

In conclusion:

Reglazing thoroughly removes all residue from the pans, and leaves the pans much cleaner than any factory personnel (or most Mashgichim) can. It is therefore a wonderful preparation for kashering.

Reglazing appears to satisfy the requirements for hag’alahb’sha’ar mashkim, which is acceptable b’dieved and not l’chathcilah.

Under Rabbinic supervision, the reglazing process can be somewhat modified to qualify as libun kal.

There is nothing in the reglazing process which even remotely resembles libun gamur.

Loaner Pans

Most companies schedule retinning and reglazing during weekends and/or when they can afford to be without specific pans (e.g. hamburger pans) for the 24-48 hours it takes from when the pans leave the facility for processing, until they return. It is noteworthy that when the processing cannot be scheduled in this manner, the reglazing or retinning company will often be able to provide “loaner pans” to the bakery (or direct them to companies who offer that service). While this is not an issue at a company which is reglazing in preparation for certification, it is a potential issue with companies that are already certified and are sending their pans out to be serviced.

Do Baking Pans Require Libun Gamur?

Baking pans are used to bake dry items directly on the fire and therefore it is clear that the correct method of kashering them is through libun gamur. Since we have seen that reglazing is not libun gamur does that mean that commercial bakeries which want to be certified must replace all of their baking pans? In practice, such a requirement will discourage most companies from seeking certification, and the question is whether there is any possibility to allow them to just kasher with libun kal.

The RCs discussed this question with Rav Schwartz, who held that in specific cases one could be lenient and rely on libun kal if (a) the pans were aino ben yomo, and (b) this was a one-time kashering. [Of course, a Mashgiach would have to be on hand to verify that the pans did in fact have a full libun kal, for we have seen that reglazing does not always qualify as libun kal.] In such cases, he said that one could rely on the following line of reasoning:7

Iggeros Moshe8 rules that the letter of the law is that a non-Jewish company may use aino ben yomo equipment without kashering, but it is incongruous (מכוער) for a hashgachah to certify such use (since a Jew would be required to kasher under those circumstances). Thus, kashering of aino ben yomo equipment at a non-Jewish company is a chumrah.

Rema 451:4 rules that any time libun gamur is only required as a chumrah, one may rely on the opinion of Haga’aos Maimonios, that what we call “libun kal” is actually the highest form of “libun” which is ever required.

1 I have not seen this process, known as hot tinning, but it is described at http://www.retinning.com/atmartha.html, and is consistent with the process described in the Poskim noted below.
Another method of retinning is through electroplating which, I believe, bonds a metal coating onto a surface using electrical charges and attraction rather than with heat. Of course, such a process does not qualify as any sort of kashering, but I understand that it is not used for commercial purposes. This is something which requires further research.

2 The Poskim are clear that the temperature required for libun gamur is absolute (תסור קליפתו, ניצוצות ניתזין, red hot) and does not fluctuate based on the temperature at which the b’liah happened (as is true of hag’alah where we apply the principle of k’bol’oh kach polto).
Some have interpreted this to mean that libun gamur requires the metal to reach an absolute temperature, in which case tin could not possibly undergo libun gamur since it cannot be heated above 450° F (its melting point). Aside from the inherent difficulty with such a conclusion (that no Poskim say that one of the metals listed in the Torah cannot undergo libun gamur), those who adopt this position would have a difficult time deciding which temperature is required because different metals (and different ores of the same metals) have radically different properties. Magen Avraham’s ruling, that liquefied tin which is just 450° F has undergone libun gamur, is implicitly rejecting the aforementioned position, and is instead saying that libun gamur requires the metal to reach an absolute state (e.g. red hot), but the temperature required for different metals to reach that state fluctuates depending on the metal ore in question.

3 It is not clear why Mishnah Berurah 451:77 records Magen Avraham’s ruling as saying that retinning causes the pan to be kashered via hag’alah. It may be that he is stressing that retinning qualifies as libun kal which takes the place of hag’alah (to which all agree), and not libun kal which takes the place of libun gamur (which is itself a machlokes, see Rema 451:4).

4 In truth, Pri Megadim does not directly say the statement quoted in the text; rather, he says that if the pan was used in a manner which demands libun gamur, then it is פשיטא that one should perform the libun before retinning (because afterwards it will not be possible), clearly implying that the retinning process itself will not qualify as a libun gamur.

7 Rav Schachter once also advanced a similar line of reasoning and added that if the only non-kosher ingredients used in the factory are gevinas akum (which is often the case if their vegetable oil is kosher), then one can also be מצטרף the (rejected) opinions of Rabbeinu Tam (in Tosfos, Avodah Zara 35a ד"ה חדא) that gevinas akum is permitted nowadays, Taz 101:4 who holds gevinas akum is only אסור מספק, and Issur V’heter (brought in Shach 115:14 and Chochmas Adam 67:5) who holds that gevinas akum is batel b’rov (in which case the baked goods made with gevinas akum are kosher b’dieved, and kashering is not required after their use).
The line of reasoning presented in the text is only valid if the company is owned by non-Jews. In situations where the company is Jewish owned, then one must consider whether there are other possible reasons to be lenient (including those noted at the beginning of this footnote), and a shailah should be asked.